The invention relates to a priming system for an internal combustion engine and a method of priming an internal combustion engine in order to facilitate easy starting of the engine.
With present day diaphragm carburetors which utilize a metering lever and fulcrum, it has proven to be difficult to adapt an efficient priming system to said carburetors. These carburetors are presently modified for priming by adding a primer fitting to the diaphragm cover, a primer line and primer pump with a primer bulb vented to atmosphere. On some engines, the bulb is sealed and the vent is in the metering diaphragm cover.
With this type of system, when the operator presses the primer bulb, the vent is closed and this pressurizes the atmospheric side of the metering diaphragm thereby pushing it against the metering lever causing the inlet valve to be opened against the metering spring which biases the valve closed. This diaphragm action forces fuel out of the metering chamber through the idle and main fuel feed orifices, into the induction tract, and further, some fuel is forced past the open inlet needle and is blocked by the outlet check valve in the fuel pump. Because the metering chamber and diaphragm are usually quite small due to size limitations on small power equipment, the total fuel displacement per diaphragm stroke will be minimal. The inlet needle valve opening is minimal due to such a small travel of the needle and as soon as the pressure drops in the metering chamber, the metering diaphragm retracts to its static position causing the inlet valve to close and shut off fuel delivery to the metering chamber.
In order for fuel to enter the metering chamber while the inlet valve is open, the fuel supply tank must be sufficiently above the carburetor for gravity to force feed the chamber. As the primer bulb is released and resumes its original shape, the atmosphere vent is uncovered and pressure is reduced at the diaphragm and in the metering chamber. This pressure reduction can aid in drawing fuel into the metering chamber, but once the diaphragm has retracted, the metering lever, following the diaphragm causes the inlet needle valve to fully close and only a small quantity of fuel will enter the chamber. This type of priming system often requires as many as twenty actuations of the manual primer to prime a dry fuel system from the tank to the carburetor, and often another six to eight primes will be required to supply enough fuel to the induction system for a cool weather engine start. This system is also ineffective when utilized with a closed fuel system wherein the fuel tank is under vacuum such as with a vacuum opening tank vent and is inoperative when the fuel tank is below the carburetor.
Attempts to prime from the inlet side of the metering valve through the metering chamber and main or idle orifices have proven to be unsatisfactory. If the cracking pressure for the metering valve is set too high, it requires a considerable amount of force on the priming bulb to overcome the resistance of the metering valve spring. On the other hand, if the cracking pressure is set too low, it may leak during normal engine operation or due to vibration thereby causing the mixture to become too rich.